17 to 18 C is just fine for me. The rest of the flats I'm in at the moment are inhabited by East Europeans who seem to prefer temperatures around 25 to 30 C. Whilst Brits reach for the woolly pully when it gets cold, Poles and Czechs reach for the thermostat. I haven't had the heating on once over the winter (well, other than a single occasion when I think I was verging on the hypothermic when I got back from an 80km one bitterly cold day) and heat from the surrounding flats has kept my flat more than warm enough, and I've been using a 4.5-tog duvet all winter. I'm not good with warm weather, and I'll suffer when summer comes around. Today was plenty warm enough for me.